On the trail of Icelandic depressions – DLR tests new laser measurement technology in flights over Greenland

Globally unique wind measurements

The wind laser measurement technology that is currently being tested and calibrated on the aircraft is an important step towards ESA's Atmospheric Dynamics Mission, ADM-Aeolus, which will be the first system in the world capable of measuring the Earth’s wind fields in detail. This new weather satellite will use a Doppler wind lidar to send wind, cloud and aerosol profiles along its path back to Earth in near real time. These profiles will be used for improving the forecasting of the development of weather systems. Researchers at the DLR Institute of Atmospheric Physics and the DLR Remote Sensing Technology Institute will process data from the satellite.

Windiest place on Earth

Europe's weather systems are formed in the arctic polar region around Iceland and Greenland. Small anomalies that occur where cold air masses from the polar regions meet warmer air masses can lead to the development of weather systems. The Icelandic depressions here are well known. In addition, the polar region of Greenland is of particular interest in climate research because of the rising temperatures in the Arctic and the associated retreat of polar ice sheets. "In the current research flight campaign, we are calibrating the new wind lidar above the extensive ice fields of Greenland – testing our algorithms in the process – to make sure that, later on, everything runs smoothly in space," says Oliver Reitebuch from the DLR Institute of Atmospheric Physics. In particular, the southern tip of Greenland – the windiest place in the world – is the perfect testing ground for the new wind measurement technology, as it is especially challenging, with pronounced tip jets and strong jet streams.

NASA and DLR joint flights

"We are conducting several flights per day above the permanent ice of Greenland and, in doing so, are acquiring comparative data from a summit station operated by our US research colleagues at an altitude of 3200 metres," says DLR test pilot Philipp Weber. After taking off from Iceland, the Falcon crew makes a refuelling stop at Kangerlussuaq in Greenland and then spends two hours criss-crossing Greenland. In total, some 10 test flights above Greenland are planned, which will mostly take place in coordination with the NASA DC-8 research aircraft. The data from the NASA DC-8 and the DLR Falcon will then be compared. Two lidar instruments are being used on board the NASA DC-8, in addition to measurement probes that are ejected from the aircraft via a chute.

Scattered light makes wind fields visible

At present, the major wind fields over the oceans are still detected optically by weather satellites tracking cloud movements, or measured indirectly using radar signals reflected by the wave motion on the surface of oceans. "The wind lidar measurements will enable us to directly measure wind speeds from ground level up to an altitude of 20 kilometres with significantly greater accuracy. Depending on the altitude, we can achieve a resolution of between 500 and 1000 metres while doing so," explains Reitebuch. "We do this by using the Doppler effect, which many people are familiar with from the siren on an ambulance." As the ambulance approaches, the sound of the siren is higher in pitch due to the Doppler effect because the wavelength of the sound is somewhat shortened. As the ambulance moves away after going past the observer, the pitch of the siren suddenly drops as the sound waves become elongated. "It is the same with Doppler lidar – we emit laser light into a wind field at a precisely defined wavelength. Depending on the movement of the wind field, the light is reflected back with a very small change in wavelength. From this, we can determine the wind speed," continues Reitebuch. Using this technology, the DLR researchers will be capable of accurately determining changes as small as one ten billionths of a wavelength.

Small anomalies with large effects on the weather

In addition to testing the wind lidar above Greenland, the DLR atmospheric researchers are acquiring data on the formation and development of Icelandic depressions. The researchers hope to better understand how low-pressure systems arise from small anomalies over Iceland, Greenland and the North Atlantic in a short time. "From Iceland, we can perform measurements in the strong jet streams over the North Atlantic. Detailed knowledge of the wind distribution is particularly important because a lack of wind data very quickly leads to errors in weather forecasting models," says Reitebuch. "These errors affect the accurate forecasting of the development of low-pressure systems, which often move towards Europe and, due to their high winds and heavy rainfall, have a significant effect on our daily lives."

About the mission